1. Field of the Invention
The invention relates to imide and pyrrolidone grease thickeners and their use with a phthalate complexing agent. More particularly the invention relates to a grease and a grease thickening system comprising (i) an imide or pyrrolidone thickener and (ii) an imide or pyrrolidone thickener in combination with a metal phthalate complexing agent, such as dilithium terephthalate.
2. Background of the Invention
Grease is a semi-fluid medium comprised of a liquid lubricant and a thickening agent. The liquid lubricant is derived from natural mineral oils, synthetic hydrocarbons, esters, ethers, polysiloxanes, fluorocarbon polymers and the like. The thickening agent is typically dispersed in the liquid lubricant at a concentration between 5 and 20 wt. %. A number of different compounds are used as grease thickeners. For example, mineral oils are thickened by alkali soaps of fatty acids, clays, polymers, phthalocyanines, organic dyes, polyureas and aluminum soaps, among others. Soap based thickeners containing alkali metal salts of natural fatty acids are widely used, particularly those of lithium, calcium and sodium. Pyrrolidone thickeners have also been used as disclosed in U.S. Pat. No. 4,253,979 the disclosure of which is incorporated herein by reference. Lithium stearates and their hydroxy-substituted derivatives appear to dominate the grease market at the present time. The use of a long chain lithium monocarboxylate together with an aliphatic dilithium dicarboxylate as a grease thickener, is known. German patent publication DT-362596 assigned to the assignee of the present invention discloses a combination of lithium 12-hydroxystearate and dilithium azealate. U.S. Pat. No. 4,897,210 discloses a dilithium salt of a terephthalate acid alkyl ester and a lithium salt of 12-hydroxystearic acid as a grease thickener. Japanese patent publication J-59157191 relates to saponifying a silicone base oil composition with lithium hydroxide, wherein the base oil contains a terephthalic acid ester such as methyl N-octadecyl terephthalamate, while Japanese patent publication J-9145297 relates to a similar process with a hydrocarbon base oil containing an N-substituted terephthalamine ester, such as N-octadecyl terephthalamine-methyl.